No you do NOT add the boxes together. You enter each W2 separately. After you enter the first one you can add another.
To add a W2 just type w2 (small W) in the search box at the top of your return and click on Jump To the W2
OR To Add a W2 enter it under
Federal Taxes on the left side
Wages & Income
Then Wages and Salaries- Click the Start or Update button
Then if you are married you should have 2 buttons...Add w-2 for (taxpayer name) and Add w-2 for (spouse name). Click on the correct button to add a W-2.